C drive will not completely defrag?

Reboot the computer and hit F8 to go to safe mode while in safe mode do disk cleanup then do defrag. if this does not help then in normal mode Click on start and run type in msconfig and then run troubleshooting reboot in trouble shoot mode. This will turn off all drivers except what is needed to operate the computer then defrag. Any anti virus programs running or firewall in the back ground will not allow you to defrag as it detects defrag as a bug that's trying to change the drive. So it won't defrag.